Epic Games Fortnite: POSTMORTEM OF SERVICE OUTAGE 4/11/2018 – 4/12/2018

Can someone from Epic explain why they're using Mongo DB despite endless horror stories about it just like this? I understand that this wasn't a bug in Mongo and caused by programmer error, but WHY mongo?

I'm not a genius DBA but I don't see postmortems like this for Cassandra, postgres, couchDB or any other databases. In fact I see the opposite: large corporations praising Cassandra et al because they made scaling work so well.